The present invention relates to mass flow controllers which control the flow of process gases into a process chamber in the fabrication of integrated circuits on semiconductor wafers in the chamber. More particularly, the present invention relates to a backfill prevention system which may be operated in conjunction with a mass flow controller to measure flow of gas through a gas flow conduit and close a valve or valves in the conduit as needed to prevent gas backfilling of the conduit.
The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.
Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate. Many of the various processing steps, including but not limited to etching and chemical vapor deposition (CVD), used in the semiconductor fabrication process require process fluids or chemicals for the formation of integrated circuits on the wafer substrate.
About 50 different types of gases are used in as many as 450 process steps in semiconductor manufacturing. Gases used in semiconductor fabrication are generally categorized as one of two types: bulk gases, which include oxygen, nitrogen, helium and argon; and specialty gases, which include chlorine and hydrogen chloride and are the process gases used to effect the circuit-fabricating chemical reactions on the semiconductor wafer substrate. Bulk gases, which include purge gases used to flush undesirable residual gases, atmospheric gases or water vapor from a process chamber, are stored in large storage tanks outside the wafer fab manufacturing area and are distributed into the proper workstation through a bulk gas distribution (BGD) system. Specialty gases are dispensed from cylinders in a gas cylinder cabinet containing a control panel. A local gas distribution system in the process area is used to deliver the gas from the cylinder to the chamber of the process tool.
The molecular quantities of the reactant gases utilized in semiconductor fabrication processes are important for proper control of the reactions. According to the ideal gas law, the number of gas molecules contained in a given volume changes in proportion to to the absolute pressure and temperature. Therefore, a given volume of gas flowing into a process chamber yields various quantities of gas molecules depending on the temperature and pressure of the gas. Accordingly, mass flow controllers (MFCs), which utilize a thermal sensor that senses the heat-transfer property of a gas to detect changes in the mass flow of the gas, are used to control the flow of gases into process chambers.
A typical conventional gas delivery system in a semiconductor fab facility is generally indicated by reference numeral 10 in FIG. 1 and includes a gas manifold 12 connected to a process chamber 40 of a process tool (not shown) in the facility. The gas manifold 12 may be contained in a valve manifold box (VMB, not shown) and includes a BCl3 gas delivery conduit 14 for conducting BCl3 to the process chamber 40, a Cl2 gas delivery conduit 15 for conducting Cl2 to the process chamber 40, an N2S gas delivery conduit 16 for conducting N2S to the process chamber 40, a CH3F gas delivery conduit 17 for conducting CH3F to the process chamber 40, and a CF4 gas delivery conduit 18 for conducting CF4 to the process chamber 40. The BCl3 and the Cl2 are each delivered to the process chamber 40 typically at a pressure of about 15 psi, whereas the N2S, the CH3F and the CF4 are delivered to the process chamber 40 typically at a pressure of about 35 psi. Each of the gas flow lines 14-18 is typically fitted with a manual valve 20 for manually opening and closing the corresponding gas flow line; a regulator 24 for controlling the gas pressure in the gas flow line; a filter 26 for filtering particles from the flowing gas; a mass flow controller (MFC) 30 for controlling the flow rate of each gas in the corresponding gas delivery conduit; and an upstream valve 28 and a downstream valve 32 on respective sides of the mass flow controller 30. The gas delivery conduits are connected to a common manifold conduit 34, from which an outlet conduit 36 conducts the gases into the process chamber 40. A final valve 38 is provided in the outlet conduit 36. The lower-pressure gas delivery conduits 14 and 15 may each be fitted with a V-block valve 22 which prevents backflow of gas through the respective gas delivery conduits.
One of the problems associated with the conventional gas delivery system 10 is that the final valve 38 frequently becomes blocked or clogged during use and is therefore incapable of opening to establish fluid communication between the manifold conduit 34 and the process chamber 40. Consequently, residual gas from the higher-pressure gas delivery conduits 16-18, such as the N2S, the CH3F or the CF4, respectively, remains in the manifold conduit 34 after flow of these gases to the process chamber 40. Accordingly, upon subsequent flow of the lower-pressure BCl3 to the process chamber 40 through the gas delivery conduit 14, the upstream valve 28 and downstream valve 32 are each opened and the higher-pressure CH3F or CF4 backflows from the manifold conduit 34 and through the downstream valve 32, the mass flow controller 30 and the upstream valve 28, respectively, of the BCl3 gas delivery conduit 14. This gas backfill causes contamination of the BCl3 gas delivery conduit 14 with the CH3F or the CF4 gas, thereby potentially adversely affecting processes carried out in process tools connected to the valve manifold box in which the gas manifold 12 is contained. Additionally, clearing of the CH3F or CF4 gas from the BCl3 gas delivery conduit 14 results in unnecessary downtime in the semiconductor processing sequence.
Accordingly, an object of the present invention is to provide a system which prevents undesired backfilling of a gas flow conduit with a gas.
Another object of the present invention is to provide a backfill prevention system which prevents gas contamination of a gas flow conduit.
Still another object of the present invention is to provide a backfill prevention system which is capable of closing a valve or valves in a gas flow conduit to prevent backfilling of the conduit with an undesired gas.
Yet another object of the present invention is to provide a backfill prevention system which prevents undesired gas contamination of a process tool for semiconductors.
A still further object of the present invention is to provide a backfill prevention system which eliminates downtime associated with clearing gas from a gas flow conduit in a semiconductor fab facility.
Yet another object of the present invention is to provide a backfill prevention system which is capable of a variety of industrial applications.
Another object of the present invention is to provide a backflow prevention system which utilizes a negative voltage signal that corresponds to reverse flow of gas in a gas flow conduit to close valves in the gas flow conduit and prevent gas backfill or contamination of the conduit.
Yet another object of the present invention is to provide a backfill prevention system which may be utilized with a mass flow controller to sense backfilling of gas in a gas flow conduit and close valves in the conduit to prevent further backfilling of the gas in the conduit.
In accordance with these and other objects and advantages, the present invention comprises a backfill prevention system for a gas flow conduit, comprising a gas flow monitor circuit which measures the rate and direction of gas flow through a gas flow conduit and converts the measured data into a voltage signal. A valve control circuit operably connected to a valve or valves in the gas flow conduit receives the voltage signal from the gas flow monitor circuit and closes the valve or valves in the event that the voltage signal indicates backflow of a gas through the gas flow conduit. The valve control conduit may further be provided with a first light emitting diode (LED) which is illuminated during normal flow of the gas through the conduit, and a second LED which is illuminated in the event of gas backflow through the conduit. The system is typically used in conjunction with a mass flow controller in the conduit.